This invention pertains to the use of a mixture of a phosphonocarboxylic acid, phytic acid, and a polyacrylic acid to inhibit formation or precipitation of alkaline earth sulfate scale in water systems where such precipitation is a particular problem. Specific applications contemplated herein include, oil field drilling, pulp and paper, and desalination.
Scale-forming salts can be prevented from precipitating by complexing the cations with chelating or sequestering agents so that the solubility of the reaction products is not exceeded. Generally, this requires stoichiometric amounts of chelating or sequestering agent with respect to the scale-forming cation, which amounts are high and are not always desirable or economical.
More than a quarter of a century ago, it was discovered that certain inorganic polyphosphates could prevent scale precipitation when added in amounts far less than the concentrations needed for sequestering or chelating. When a precipitation inhibitor is present in a potentially scale-forming system at a markedly lower concentration than that required for sequestering the scale-forming cation, it is said to be present in a "threshold" amount. Threshold inhibition describes the phenomenon whereby a substoichiometric amount of a scale inhibitor can stabilize a solution from precipitation, which solution can contain hundreds or thousands of parts of scale-forming ions. Threshold inhibition generally takes place under conditions where a few, i.e., 1 to 10 ppm, of a polymeric inhibitor will stabilize in solution from about 100 to several thousand ppm of a scale-forming mineral.
As already discussed above, whereas threshold inhibition occurs at substoichiometric ratios of inhibitor to scale-forming cation, sequestration requires a stoichiometric ratio of sequestrant to scale-forming cation to maintain that cation in solution. Generally, sequestering takes place at a weight ratio of threshold active compound to scale-forming cation components of greater than about ten to one, depending on the anion components in the water. Threshold inhibition, however, generally takes place at a weight ratio of threshold active compound to scale forming cation components of less than about 0.5 to 1.0.
Therefore, on the basis of the above discussions, the tremendous difference between sequestration and threshold inhibition reflects the obvious advantages of the latter over the former.
U.S. Pat. No. 4,452,703 to Ralston et al describes scale inhibition of calcium phosphate and calcium hydroxide in sugar evaporation equipment by the use of a scale inhibition composition at a level of 0.1 to 200 ppm. Scale inhibitors disclosed by this patent include polymaleic acid, amine adducts of maleic anhydride polymers, phosphonobutane tricarboxylic acid, phosphinocarboxylic acids, and copolymers of an acrylic acid and a hydroxylated lower alkyl acrylate.
U.S. Pat. No. 4,386,005 to Kapiloff et al discloses the use of a mixture of polyacrylic acid and phytic acid at a level of 0.01 to 20 ppm to inhibit scaling of calcium, magnesium, and/or iron scales. The ingredients are used in the relative ratio of 1 to 0.05 part by weight of polyacrylic acid and 0.05 to 1 part by weight of phytic acid. Polyacrylic acid is of a low molecular, ranging from about 1,000 to about 10,000.